The value of Magnetic Resonance Imaging (MRI) devices for medical use was recognized almost immediately after they first appeared in the 1970s. Because they appear both to do no harm to the human body, and to create better images of the body's interior than the best X-ray technology, they have gained widespread use for diagnosis, pre-operative examination and even for assistance during surgical procedures. While MRI provides information on size and location of pathological abnormalities such as tumors, a variation of magnetic resonance technology called Magnetic Resonance Spectroscopy (MRS)—which identifies various biochemicals and their concentrations—often can help further by providing more information on the tissue chemistry of the target abnormality.
The typical MRI/MRS process consists first in the magnetic alignment of nuclei of a particular target nuclear species by a static main magnetic field produced by a solenoid magnet apparatus, arranged so that the cylindrical space bounded by the solenoid windings (i.e. the main magnet bore) forms a convenient space and platform for placement of an object containing the target nuclei. This application of the main magnetic field is followed by a sequence of pulses of a second magnetic field, by means of a RF (Radio Frequency) pulse generator, RF transmitter amplifier, and RF volume resonator (i.e. near-field transmit antenna). The result of this sequential pulsing is a perturbed magnetization of the target nuclei, transverse to the main field, in which condition the perturbed magnetization precesses about the main field. The Larmor frequency of the target nuclear species—the characteristic frequency of precession of target nuclei at a given strength of the main magnetic field—is linearly proportional to the strength of the main magnetic field. During precession, the magnetization is further perturbed by a sequence of magnetic gradient pulses, applied by means of pulse generators and gradient induction coils, which produce variations of the main magnetic field strength. At pre-selected intervals throughout the precession period, RF signals arising from the precessing target nuclei are detected at their characteristic frequencies by a tuned RF near-field receive antenna and the signals are fed through an RF receiver and image processing apparatus in order to compute and display images and/or spectra of the target nuclei.
The clinical use MRI/MRS technology is based on detecting and interpreting radio frequency RF excitation originating from target atomic nuclei in human tissue in response to manipulation of those nuclei with magnetic fields in a manner similar to that outlined above.
The distance from a large radio-frequency near-field antenna (or ‘coil’ as it is known in the art) to the tissue in question can be so great as to render said coil unable to provide the necessary strength of signal required for the level of detail and accuracy needed for proper evaluation of the target tissue. A greater distance means a weaker signal. Yet, the strength of the RF signal received from the nuclei is of fundamental importance both to the accuracy of the details of the image produced and the information on tissue chemistry.
Ongoing research and development of MRI/MRS technology has sought to use smaller ‘local’ coils, placed nearer the location of the tissue in question so as improve the strength and quality of information carried by the RF signals generated by the tissue. One focus of this effort with local coils has been the endo-rectal coils used to analyze pelvic pathologies including prostate cancer. And while gains have been made, research for over 15 years has left significant room for improvement in the use of endo-rectal devices.
One particular need is to take advantage of the opportunities offered by dual-testing of more than one species of target nuclei during a single examination. In particular, a dual-test of the tissue in question using both proton and carbon-13 nuclei could greatly improve results.
The most common magnetic nucleus targeted in clinical MRI/MRS is a proton, that is to say, an ordinary hydrogen nucleus, which is valuable by virtue of its high abundance and nearly universal distribution in biological tissue, as well for the high intrinsic RF signal obtained from its large magnetic moment during the test procedure.
An excellent choice for improving the MRI/MRS analysis of the chemistry of the tissue is found in the nuclei of an isotope of carbon, carbon-13, which can be used to trace a multitude of metabolic processes and transformations in normal and diseased states. Although of very limited natural abundance and possessing a weak magnetic moment, recent advances in the technology of hyperpolarization, can enhance the magnetization of carbon-13, and, to make it available in the test tissue, it can be introduced into an imaging subject by the use of an exogenous bolus of a metabolite compound enriched in carbon-13 and subjected to hyperpolarization. The resulting strong exogenous signal overwhelms the endogenous carbon background signal in the body, and permits the tracing of sequential metabolic transformations, in healthy or diseased tissue.
But present endo-rectal technology can not reliably make use of two such signals—proton and carbon-13—during the same test. One issue is the signal quality limitations resulting from the circuitry design of today's endo-rectal devices, which cannot adequately block the wrong RF signals while gathering the right ones. The design of the body of the apparatus is also a source of error. The endo-rectal design commonly used at present for MRI/MRS examination of pelvic tissue abnormalities such as prostate cancer has an inflatable, or balloon, body, which encloses a large volume of air. This creates a large discontinuity in static magnetic susceptibility at the interface between antenna body and tissue, resulting in degradation of image quality and spectral quality.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for improved reception of RF signals originating from tissue in response to magnetic fields generated by MRI/MRS technology. This means a continued need to place the antenna close to the prostate, while improving the general quality of RF signal reception from one or more species of target nuclei. Such an improvement would be especially valuable for the use of carbon-13 in MRI/MRS.
The above-mentioned shortcomings, disadvantages and problems are addressed herein, which will be understood by reading and studying the following specification.